System constraints method of controlling operation of an electro-mechanical transmission with two external input torque ranges

ABSTRACT

A method to control an electro-mechanical transmission mechanically-operatively coupled to an internal combustion engine and first and second electric machines to transmit power to an output member includes determining motor torque constraints and battery power constraints. A preferred output torque to an output member is determined that is achievable within the motor torque constraints and is achievable within a range for a first torque input and is achievable within a range for a second torque input and is based upon the battery power constraints.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 60/984,830 filed on Nov. 2, 2007 which is hereby incorporated herein by reference.

TECHNICAL FIELD

This disclosure pertains to control systems for electro-mechanical transmissions.

BACKGROUND

The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.

Known hybrid powertrain architectures can include multiple torque-generative devices, including internal combustion engines and non-combustion machines, e.g., electric machines, which transmit torque through a transmission device to an output member. One exemplary hybrid powertrain includes a two-mode, compound-split, electro-mechanical transmission which utilizes an input member for receiving tractive torque from a prime mover power source, preferably an internal combustion engine, and an output member. The output member can be operatively connected to a driveline for a motor vehicle for transmitting tractive torque thereto. Machines, operative as motors or generators, can generate torque inputs to the transmission independently of a torque input from the internal combustion engine. The machines may transform vehicle kinetic energy transmitted through the vehicle driveline to energy that is storable in an energy storage device. A control system monitors various inputs from the vehicle and the operator and provides operational control of the hybrid powertrain, including controlling transmission operating state and gear shifting, controlling the torque-generative devices, and regulating the power interchange among the energy storage device and the machines to manage outputs of the transmission, including torque and rotational speed.

SUMMARY

A powertrain includes an electro-mechanical transmission mechanically-operatively coupled to an internal combustion engine and first and second electric machines to transmit power to an output member. A method for controlling the electro-mechanical transmission includes determining motor torque constraints for the first and second electric machines, and determining battery power constraints for an electrical energy storage device electrically connected to the first and second electric machines. A range for a first torque input to the electro-mechanical transmission is determined and a range for a second torque input to the electro-mechanical transmission is determined. A preferred output torque to the output member of the electro-mechanical transmission is determined that is achievable within the motor torque constraints, is achievable within the range for the first torque input, is achievable within the range for the second torque input, and is based upon the battery power constraints.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments will now be described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of an exemplary powertrain, in accordance with the present disclosure;

FIG. 2 is a schematic diagram of an exemplary architecture for a control system and powertrain, in accordance with the present disclosure; and

FIGS. 3-5 are graphical diagrams, in accordance with the present disclosure.

DETAILED DESCRIPTION

Referring now to the drawings, wherein the showings are for the purpose of illustrating certain exemplary embodiments only and not for the purpose of limiting the same, FIGS. 1 and 2 depict an exemplary electro-mechanical hybrid powertrain. The exemplary electro-mechanical hybrid powertrain in accordance with the present disclosure is depicted in FIG. 1, comprising a two-mode, compound-split, electro-mechanical hybrid transmission 10 operatively connected to an engine 14 and first and second electric machines (‘MG-A’) 56 and (‘MG-B’) 72. The engine 14 and first and second electric machines 56 and 72 each generate power which can be transferred to the transmission 10. The power generated by the engine 14 and the first and second electric machines 56 and 72 and transferred to the transmission 10 is described in terms of input and motor torques, referred to herein as T_(I), T_(A), and T_(B) respectively, and speed, referred to herein as N_(I), N_(A), and N_(B), respectively.

The exemplary engine 14 comprises a multi-cylinder internal combustion engine selectively operative in several states to transfer torque to the transmission 10 via an input shaft 12, and can be either a spark-ignition or a compression-ignition engine. The engine 14 includes a crankshaft (not shown) operatively coupled to the input shaft 12 of the transmission 10. A rotational speed sensor 11 monitors rotational speed of the input shaft 12. Power output from the engine 14, comprising rotational speed and engine torque, can differ from the input speed N_(I) and the input torque T_(I) to the transmission 10 due to placement of torque-consuming components on the input shaft 12 between the engine 14 and the transmission 10, e.g., a hydraulic pump (not shown) and/or a torque management device (not shown).

The exemplary transmission 10 comprises three planetary-gear sets 24, 26 and 28, and four selectively engageable torque-transferring devices, i.e., clutches C1 70, C2 62, C3 73, and C4 75. As used herein, clutches refer to any type of friction torque transfer device including single or compound plate clutches or packs, band clutches, and brakes, for example. A hydraulic control circuit 42, preferably controlled by a transmission control module (hereafter ‘TCM’) 17, is operative to control clutch states. Clutches C2 62 and C4 75 preferably comprise hydraulically-applied rotating friction clutches. Clutches C1 70 and C3 73 preferably comprise hydraulically-controlled stationary devices that can be selectively grounded to a transmission case 68. Each of the clutches C1 70, C2 62, C3 73, and C4 75 is preferably hydraulically applied, selectively receiving pressurized hydraulic fluid via the hydraulic control circuit 42.

The first and second electric machines 56 and 72 preferably comprise three-phase AC machines, each including a stator (not shown) and a rotor (not shown), and respective resolvers 80 and 82. The motor stator for each machine is grounded to an outer portion of the transmission case 68, and includes a stator core with coiled electrical windings extending therefrom. The rotor for the first electric machine 56 is supported on a hub plate gear that is operatively attached to shaft 60 via the second planetary gear set 26. The rotor for the second electric machine 72 is fixedly attached to a sleeve shaft hub 66.

Each of the resolvers 80 and 82 preferably comprises a variable reluctance device including a resolver stator (not shown) and a resolver rotor (not shown). The resolvers 80 and 82 are appropriately positioned and assembled on respective ones of the first and second electric machines 56 and 72. Stators of respective ones of the resolvers 80 and 82 are operatively connected to one of the stators for the first and second electric machines 56 and 72. The resolver rotors are operatively connected to the rotor for the corresponding first and second electric machines 56 and 72. Each of the resolvers 80 and 82 is signally and operatively connected to a transmission power inverter control module (hereafter ‘TPIM’) 19, and each senses and monitors rotational position of the resolver rotor relative to the resolver stator, thus monitoring rotational position of respective ones of first and second electric machines 56 and 72. Additionally, the signals output from the resolvers 80 and 82 are interpreted to provide the rotational speeds for first and second electric machines 56 and 72, i.e., N_(A) and N_(B), respectively.

The transmission 10 includes an output member 64, e.g. a shaft, which is operably connected to a driveline 90 for a vehicle (not shown), to provide output power to the driveline 90 that is transferred to vehicle wheels 93, one of which is shown in FIG. 1. The output power at the output member 64 is characterized in terms of an output rotational speed N_(O) and an output torque T_(O). A transmission output speed sensor 84 monitors rotational speed and rotational direction of the output member 64. Each of the vehicle wheels 93 is preferably equipped with a sensor 94 adapted to monitor wheel speed, the output of which is monitored by a control module of a distributed control module system described with respect to FIG. 2, to determine vehicle speed, and absolute and relative wheel speeds for braking control, traction control, and vehicle acceleration management.

The input torque from the engine 14 and the motor torques from the first and second electric machines 56 and 72 (T_(I), T_(A), and T_(B) respectively) are generated as a result of energy conversion from fuel or electrical potential stored in an electrical energy storage device (hereafter ‘ESD’) 74. The ESD 74 is high voltage DC-coupled to the TPIM 19 via DC transfer conductors 27. The transfer conductors 27 include a contactor switch 38. When the contactor switch 38 is closed, under normal operation, electric current can flow between the ESD 74 and the TPIM 19. When the contactor switch 38 is opened electric current flow between the ESD 74 and the TPIM 19 is interrupted. The TPIM 19 transmits electrical power to and from the first electric machine 56 by transfer conductors 29, and the TPIM 19 similarly transmits electrical power to and from the second electric machine 72 by transfer conductors 31 to meet the torque commands for the first and second electric machines 56 and 72 in response to the motor torques T_(A) and T_(B). Electrical current is transmitted to and from the ESD 74 in accordance with whether the ESD 74 is being charged or discharged.

The TPIM 19 includes the pair of power inverters (not shown) and respective motor control modules (not shown) configured to receive the torque commands and control inverter states therefrom for providing motor drive or regeneration functionality to meet the commanded motor torques T_(A) and T_(B). The power inverters comprise known complementary three-phase power electronics devices, and each includes a plurality of insulated gate bipolar transistors (not shown) for converting DC power from the ESD 74 to AC power for powering respective ones of the first and second electric machines 56 and 72, by switching at high frequencies. The insulated gate bipolar transistors form a switch mode power supply configured to receive control commands. There is typically one pair of insulated gate bipolar transistors for each phase of each of the three-phase electric machines. States of the insulated gate bipolar transistors are controlled to provide motor drive mechanical power generation or electric power regeneration functionality. The three-phase inverters receive or supply DC electric power via DC transfer conductors 27 and transform it to or from three-phase AC power, which is conducted to or from the first and second electric machines 56 and 72 for operation as motors or generators via transfer conductors 29 and 31 respectively.

FIG. 2 is a schematic block diagram of the distributed control module system. The elements described hereinafter comprise a subset of an overall vehicle control architecture, and provide coordinated system control of the exemplary hybrid powertrain described in FIG. 1. The distributed control module system synthesizes pertinent information and inputs, and executes algorithms to control various actuators to meet control objectives, including objectives related to fuel economy, emissions, performance, drivability, and protection of hardware, including batteries of ESD 74 and the first and second electric machines 56 and 72. The distributed control module system includes an engine control module (hereafter ‘ECM’) 23, the TCM 17, a battery pack control module (hereafter ‘BPCM’) 21, and the TPIM 19. A hybrid control module (hereafter ‘HCP’) 5 provides supervisory control and coordination of the ECM 23, the TCM 17, the BPCM 21, and the TPIM 19. A user interface (‘UI’) 13 is signally connected to a plurality of devices through which a vehicle operator controls or directs operation of the electro-mechanical hybrid powertrain. The devices include an accelerator pedal 113 (‘AP’), an operator brake pedal 112 (‘BP’), a transmission gear selector 114 (‘PRNDL’), and a vehicle speed cruise control (not shown). The transmission gear selector 114 may have a discrete number of operator-selectable positions, including the rotational direction of the output member 64 to enable one of a forward and a reverse direction.

The aforementioned control modules communicate with other control modules, sensors, and actuators via a local area network (hereafter ‘LAN’) bus 6. The LAN bus 6 allows for structured communication of states of operating parameters and actuator command signals between the various control modules. The specific communication protocol utilized is application-specific. The LAN bus 6 and appropriate protocols provide for robust messaging and multi-control module interfacing between the aforementioned control modules, and other control modules providing functionality including e.g., antilock braking, traction control, and vehicle stability. Multiple communications buses may be used to improve communications speed and provide some level of signal redundancy and integrity. Communication between individual control modules can also be effected using a direct link, e.g., a serial peripheral interface (‘SPI’) bus (not shown).

The HCP 5 provides supervisory control of the hybrid powertrain, serving to coordinate operation of the ECM 23, TCM 17, TPIM 19, and BPCM 21. Based upon various input signals from the user interface 13 and the hybrid powertrain, including the ESD 74, the HCP 5 determines an operator torque request, an output torque command, an engine input torque command, clutch torque(s) for the applied torque-transfer clutches C1 70, C2 62, C3 73, C4 75 of the transmission 10, and the motor torques T_(A) and T_(B) for the first and second electric machines 56 and 72. The TCM 17 is operatively connected to the hydraulic control circuit 42 and provides various functions including monitoring various pressure sensing devices (not shown) and generating and communicating control signals to various solenoids (not shown) thereby controlling pressure switches and control valves contained within the hydraulic control circuit 42.

The ECM 23 is operatively connected to the engine 14, and functions to acquire data from sensors and control actuators of the engine 14 over a plurality of discrete lines, shown for simplicity as an aggregate bi-directional interface cable 35. The ECM 23 receives the engine input torque command from the HCP 5. The ECM 23 determines the actual engine input torque, T_(I), provided to the transmission 10 at that point in time based upon monitored engine speed and load, which is communicated to the HCP 5. The ECM 23 monitors input from the rotational speed sensor 11 to determine the engine input speed to the input shaft 12, which translates to the transmission input speed, N_(I). The ECM 23 monitors inputs from sensors (not shown) to determine states of other engine operating parameters including, e.g., a manifold pressure, engine coolant temperature, ambient air temperature, and ambient pressure. The engine load can be determined, for example, from the manifold pressure, or alternatively, from monitoring operator input to the accelerator pedal 113. The ECM 23 generates and communicates command signals to control engine actuators, including, e.g., fuel injectors, ignition modules, and throttle control modules, none of which are shown.

The TCM 17 is operatively connected to the transmission 10 and monitors inputs from sensors (not shown) to determine states of transmission operating parameters. The TCM 17 generates and communicates command signals to control the transmission 10, including controlling the hydraulic circuit 42. Inputs from the TCM 17 to the HCP 5 include estimated clutch torques for each of the clutches, i.e., C1 70, C2 62, C3 73, and C4 75, and rotational output speed, N_(O), of the output member 64. Other actuators and sensors may be used to provide additional information from the TCM 17 to the HCP 5 for control purposes. The TCM 17 monitors inputs from pressure switches (not shown) and selectively actuates pressure control solenoids (not shown) and shift solenoids (not shown) of the hydraulic circuit 42 to selectively actuate the various clutches C1 70, C2 62, C3 73, and C4 75 to achieve various transmission operating range states, as described hereinbelow.

The BPCM 21 is signally connected to sensors (not shown) to monitor the ESD 74, including states of electrical current and voltage parameters, to provide information indicative of parametric states of the batteries of the ESD 74 to the HCP 5. The parametric states of the batteries preferably include battery state-of-charge, battery voltage, battery temperature, and available battery power, referred to as a range P_(BAT) _(—) _(MIN) to P_(BAT) _(—) _(MAX).

A brake control module (hereafter ‘BrCM’) 22 is operatively connected to friction brakes (not shown) on each of the vehicle wheels 93. The BrCM 22 monitors the operator input to the brake pedal 112 and generates control signals to control the friction brakes and sends a control signal to the HCP 5 to operate the first and second electric machines 56 and 72 based thereon.

Each of the control modules ECM 23, TCM 17, TPIM 19, BPCM 21, and BrCM 22 is preferably a general-purpose digital computer comprising a microprocessor or central processing unit, storage mediums comprising read only memory (‘ROM’), random access memory (‘RAM’), electrically programmable read only memory (‘EPROM’), a high speed clock, analog to digital (‘A/D’) and digital to analog (‘D/A’) circuitry, and input/output circuitry and devices (‘I/O’) and appropriate signal conditioning and buffer circuitry. Each of the control modules has a set of control algorithms, comprising resident program instructions and calibrations stored in one of the storage mediums and executed to provide the respective functions of each computer. Information transfer between the control modules is preferably accomplished using the LAN bus 6 and serial peripheral interface buses. The control algorithms are executed during preset loop cycles such that each algorithm is executed at least once each loop cycle. Algorithms stored in the non-volatile memory devices are executed by one of the central processing units to monitor inputs from the sensing devices and execute control and diagnostic routines to control operation of the actuators, using preset calibrations. Loop cycles are executed at regular intervals, for example each 3.125, 6.25, 12.5, 25 and 100 milliseconds during ongoing operation of the hybrid powertrain. Alternatively, algorithms may be executed in response to the occurrence of an event.

The exemplary hybrid powertrain selectively operates in one of several operating range states that can be described in terms of an engine state comprising one of an engine-on state (‘ON’) and an engine-off state (‘OFF’), and a transmission state comprising a plurality of fixed gears and continuously variable operating modes, described with reference to Table 1, below.

TABLE 1 Engine Transmission Operating Applied Description State Range State Clutches M1_Eng_Off OFF EVT Mode 1 C1 70 M1_Eng_On ON EVT Mode 1 C1 70 G1 ON Fixed Gear Ratio 1 C1 70 C4 75 G2 ON Fixed Gear Ratio 2 C1 70 C2 62 M2_Eng_Off OFF EVT Mode 2 C2 62 M2_Eng_On ON EVT Mode 2 C2 62 G3 ON Fixed Gear Ratio 3 C2 62 C4 75 G4 ON Fixed Gear Ratio 4 C2 62 C3 73

Each of the transmission operating range states is described in the table and indicates which of the specific clutches C1 70, C2 62, C3 73, and C4 75 are applied for each of the operating range states. A first continuously variable mode, i.e., EVT Mode 1, or M1, is selected by applying clutch C1 70 only in order to “ground” the outer gear member of the third planetary gear set 28. The engine state can be one of ON (‘M1_Eng_On’) or OFF (‘M1_Eng_Off’). A second continuously variable mode, i.e., EVT Mode 2, or M2, is selected by applying clutch C2 62 only to connect the shaft 60 to the carrier of the third planetary gear set 28. The engine state can be one of ON (‘M2_Eng_On’) or OFF (‘M2_Eng_Off’). For purposes of this description, when the engine state is OFF, the engine input speed is equal to zero revolutions per minute (‘RPM’), i.e., the engine crankshaft is not rotating. A fixed gear operation provides a fixed ratio operation of input-to-output speed of the transmission 10, i.e., N_(I)/N_(O). A first fixed gear operation (‘G1’) is selected by applying clutches C1 70 and C4 75. A second fixed gear operation (‘G2’) is selected by applying clutches C1 70 and C2 62. A third fixed gear operation (‘G3’) is selected by applying clutches C2 62 and C4 75. A fourth fixed gear operation (‘G4’) is selected by applying clutches C2 62 and C3 73. The fixed ratio operation of input-to-output speed increases with increased fixed gear operation due to decreased gear ratios in the planetary gears 24, 26, and 28. The rotational speeds of the first and second electric machines 56 and 72, N_(A) and N_(B) respectively, are dependent on internal rotation of the mechanism as defined by the clutching and are proportional to the input speed measured at the input shaft 12.

In response to operator input via the accelerator pedal 113 and brake pedal 112 as captured by the user interface 13, the HCP 5 and one or more of the other control modules determine torque commands to control the torque generative devices comprising the engine 14 and first and second electric machines 56 and 72 to meet the operator torque request at the output member 64 and transferred to the driveline 90. Based upon input signals from the user interface 13 and the hybrid powertrain including the ESD 74, the HCP 5 determines the operator torque request, a commanded output torque between the transmission 10 and the driveline 90, an input torque from the engine 14, clutch torques for the torque-transfer clutches C1 70, C2 62, C3 73, C4 75 of the transmission 10; and the motor torques for the first and second electric machines 56 and 72, respectively, as is described hereinbelow. The commanded output torque can be a tractive torque wherein torque flow originates in the engine 14 and the first and second electric machines 56 and 72 and is transferred through the transmission 10 to the driveline 90, and can be a reactive torque wherein torque flow originates in the vehicle wheels 93 of the driveline 90 and is transferred through the transmission 10 to first and second electric machines 56 and 72 and the engine 14.

Final vehicle acceleration can be affected by other factors including, e.g., road load, road grade, and vehicle mass. The operating range state is determined for the transmission 10 based upon a variety of operating characteristics of the hybrid powertrain. This includes the operator torque request communicated through the accelerator pedal 113 and brake pedal 112 to the user interface 13 as previously described. The operating range state may be predicated on a hybrid powertrain torque demand caused by a command to operate the first and second electric machines 56 and 72 in an electrical energy generating mode or in a torque generating mode. The operating range state can be determined by an optimization algorithm or routine which determines optimum system efficiency based upon operator demand for power, battery state of charge, and energy efficiencies of the engine 14 and the first and second electric machines 56 and 72. The control system manages torque inputs from the engine 14 and the first and second electric machines 56 and 72 based upon an outcome of the executed optimization routine, and system efficiencies are optimized thereby, to manage fuel economy and battery charging. Furthermore, operation can be determined based upon a fault in a component or system. The HCP 5 monitors the torque-generative devices, and determines the power output from the transmission 10 required in response to the desired output torque at output member 64 to meet the operator torque request. As should be apparent from the description above, the ESD 74 and the first and second electric machines 56 and 72 are electrically-operatively coupled for power flow therebetween. Furthermore, the engine 14, the first and second electric machines 56 and 72, and the electro-mechanical transmission 10 are mechanically-operatively coupled to transfer power therebetween to generate a power flow to the output member 64.

Operation of the engine 14 and transmission 10 is constrained by power, torque and speed limits of the engine 14, the first and second electric machines 56 and 72, the ESD 74 and the clutches C1 70, C2 62, C3 73, and C4 75. The operating constraints on the engine 14 and transmission 10 can be translated to a set of system constraint equations executed as one or more algorithms in one of the control modules, e.g., the HCP 5.

Referring again to FIG. 1, in overall operation, the transmission 10 operates in one of the operating range states through selective actuation of one or two of the torque-transfer clutches. Torque constraints for each of the engine 14 and the first and second electric machines 56 and 72 and speed constraints for each of the engine 14, the first and second electric machines 56 and 72, and the output shaft 64 of the transmission 10 are determined. Battery power constraints for the ESD 74 are determined, and are applied to further limit the motor torque constraints for the first and second electrical machines 56 and 72. The preferred operating region for the powertrain is determined using the system constraint equation, based upon the battery power constraints, the motor torque constraints, and the speed constraints. The preferred operating region comprises a range of permissible operating torques or speeds for the engine 14 and the first and second electric machines 56 and 72.

By deriving and simultaneously solving dynamics equations of the transmission 10, the torque limit, in this embodiment the output torque T_(O), can be determined using the following linear equations:

T _(M1) =T _(A) to T _(M1) *T _(A) +T _(B) to T _(M1) *T _(B)+Misc_(—) T _(M1)   [1]

T _(M2) =T _(A) to T _(M2) *T _(A) +T _(B) to T _(M2) *T _(B)+Misc_(—) T _(M2)   [2]

T _(M3) =T _(A) to T _(M3) *T _(A) +T _(B) to T _(M3) *T _(B)+Misc_(—) T _(M3)   [3]

wherein, in this embodiment,

-   -   T_(M1) represents the output torque T_(O) at output member 64,     -   T_(M2) represents the input torque T₁ at input shaft 12,     -   T_(M3) represents the reactive clutch torque(s) for the applied         torque-transfer clutches C1 70, C2 62, C3 73, C4 75 of the         transmission 10,     -   T_(A) to T_(M1), T_(A) to T_(M2), T_(A) to T_(M3) are         contributing factors of T_(A) to T_(M1), T_(M2), T_(M3),         respectively,     -   T_(B) to T_(M1), T_(B) to T_(M2), T_(B) to T_(M3) are         contributing factors of T_(B) to T_(M1), T_(M2), T_(M3),         respectively,     -   Misc_T_(M1), Misc_T_(M2), and Misc_T_(M3) are constants which         contribute to T_(M1), T_(M2), T_(M3) by N_(I) _(—) _(DOT), N_(O)         _(—) _(DOT), and N_(C) _(—) _(DOT) (time-rate changes in the         input speed, output speed and clutch slip speed) respectively,         and     -   T_(A) and T_(B) are the motor torques from the first and second         electric machines 56 and 72.         The torque parameters T_(M1), T_(M2), T_(M3) can be any three         independent parameters, depending upon the application.

The engine 14 and transmission 10 and the first and second electric machines 56 and 72 have speed constraints, torque constraints, and battery power constraints due to mechanical and system limitations.

The speed constraints can include engine speed constraints of N_(I)=0 (engine off state), and N_(I) ranging from 600 rpm (idle) to 6000 rpm for the engine 14. The speed constraints for the first and second electric machines 56 and 72 can be as follows:

−10,500 rpm≦N_(A)≦+10,500 rpm, and

−10,500 rpm≦N_(B)≦+10,500 rpm.

The torque constraints include engine torque constraints including T_(I) _(—) _(MIN)<T_(I)<T_(I) _(—) _(MAX), and motor torque constraints for the first and second electric machines including T_(A) _(—) _(MIN)<T_(A)<T_(A) _(—) _(MAX) and T_(B) _(—) _(MIN)<T_(B)<T_(B) _(—) _(MAX). The motor torque constraints T_(A) _(—) _(MAX) and T_(A) _(—) _(MIN) comprise torque limits for the first electric machine 56 when working as a torque-generative motor and an electrical generator, respectively. The motor torque constraints T_(B) _(—) _(MAX) and T_(B) _(—) _(MIN) comprise torque limits for the second electric machine 72 when working as a torque-generative motor and an electrical generator, respectively. The maximum and minimum motor torque constraints T_(A) _(—) _(MAX), T_(A) _(—) _(MIN), T_(B) _(—) _(MAX), T_(B) _(—) _(MIN) are preferably obtained from data sets stored in tabular format within one of the memory devices of one of the control modules. Such data sets are empirically derived from conventional dynamometer testing of the combined motor and power electronics (e.g., power inverter) at various temperature and voltage conditions.

Battery power constraints comprise the available battery power within the range of P_(BAT) _(—) _(MIN) to P_(BAT) _(—) _(MAX), wherein P_(BAT) _(—) _(MIN) is maximum allowable battery charging power and P_(BAT) _(—) _(MAX) is the maximum allowable battery discharging power. Battery power is defined as positive when discharging and negative when charging.

Minimum and maximum values for T_(M1) are determined within the speed constraints, the motor torque constraints, clutch torque constraints, and the battery power constraints during ongoing operation, in order to control operation of the engine 14, the first and second electric machines 56 and 72, also referred to hereinafter as Motor A 56 and Motor B 72, and the transmission 10 to meet the operator torque request and the commanded output torque.

An operating range, comprising a torque output range is determinable based upon the battery power constraints of the ESD 74. Calculation of battery power usage, P_(BAT) is as follows:

P _(BAT) =P _(A,ELEC) +P _(B,ELEC) +P _(DC) _(—) _(LOAD)   [4]

wherein P_(A,ELEC) comprises electrical power from Motor A 56,

P_(B,ELEC) comprises electrical power from Motor B 72, and

P_(DC) _(—) _(LOAD) comprises known DC load, including accessory loads.

Substituting equations for P_(A,ELEC) and P_(B,ELEC), yields the following:

P _(BAT)=(P _(A,MECH) +P _(A,LOSS))+(P _(B,MECH) +P _(B,LOSS))+P _(DC) _(—) _(LOAD)   [5]

wherein P_(A,MECH) comprises mechanical power from Motor A 56,

P_(A,LOSS) comprises power losses from Motor A 56,

P_(B,MECH) comprises mechanical power from Motor B 72, and

P_(B,LOSS) comprises power losses from Motor B 72.

Eq. 5 can be restated as Eq. 6, below, wherein speeds, N_(A) and N_(B), and torques, T_(A) and T_(B), are substituted for powers P_(A) and P_(B). This includes an assumption that motor and inverter losses can be mathematically modeled as a quadratic equation based upon torque as follows:

$\begin{matrix} {P_{BAT} = {\left( {{N_{A}T_{A}} + \left( {{{a_{1}\left( N_{A} \right)}T_{A}^{2}} + {{a_{2}\left( N_{A} \right)}T_{A}} + {a_{3}\left( N_{A} \right)}} \right)} \right) + \left( {{N_{B}T_{B}} + \left( {{{b_{1}\left( N_{B} \right)}T_{B}^{2}} + {{b_{2}\left( N_{B} \right)}T_{B}} + {b_{3}\left( N_{B} \right)}} \right)} \right) + P_{DC\_ LOAD}}} & \lbrack 6\rbrack \end{matrix}$

wherein N_(A), N_(B) comprise speeds of Motors A and B 56 and 72,

-   -   T_(A), T_(B) comprise torques of Motors A and B 56 and 72, and     -   a1, a2, a3, b1, b2, b3 each comprise quadratic coefficients         which are a function of respective motor speeds, N_(A), N_(B).

This can be restated as Eq. 7 as follows.

$\begin{matrix} {P_{BAT} = {{a_{1}*T_{A}^{2}} + {\left( {N_{A} + a_{2}} \right)*T_{A}} + {b_{1}*T_{B}^{2}} + {\left( {N_{B} + b_{2}} \right)*T_{B}} + {a\; 3} + {b\; 3} + P_{DC\_ LOAD}}} & \lbrack 7\rbrack \end{matrix}$

This reduces to Eq. 8 as follows.

$\begin{matrix} {P_{BAT} = {{a_{1}\left\lbrack {T_{A}^{2} + {{T_{A}\left( {N_{A} + a_{2}} \right)}/a_{1}} + \left( {\left( {N_{A} + a_{2}} \right)/\left( {2*a_{1}} \right)} \right)^{2}} \right\rbrack} + {b_{1}\left\lbrack {T_{B}^{2} + {{T_{B}\left( {N_{B} + b_{2}} \right)}/b_{1}} + \left( {\left( {N_{B} + b_{2}} \right)/\left( {2*b_{1}} \right)} \right)^{2}} \right\rbrack} + {a\; 3} + {b\; 3} + P_{DC\_ LOAD} - {\left( {N_{A} + a_{2}} \right)^{2}/\left( {4*a_{1}} \right)} - {\left( {N_{B} + b_{2}} \right)^{2}/\left( {4*b_{1}} \right)}}} & \lbrack 8\rbrack \end{matrix}$

This reduces to Eq. 9 as follows.

$\begin{matrix} {P_{BAT} = {{a_{1}\left\lbrack {T_{A} + {\left( {N_{A} + a_{2}} \right)/\left( {2*a_{1}} \right)}} \right\rbrack}^{2} + {b_{1}\left\lbrack {T_{B} + {\left( {N_{B} + b_{2}} \right)/\left( {2*b_{1}} \right)}} \right\rbrack}^{2} + a_{3} + b_{3} + P_{DC\_ LOAD} - {\left( {N_{A} + a_{2}} \right)^{2}/\left( {4*a_{1}} \right)} - {\left( {N_{B} + b_{2}} \right)^{2}/\left( {4*b_{1}} \right)}}} & \lbrack 9\rbrack \end{matrix}$

This reduces to Eq. 10 as follows.

$\begin{matrix} {P_{BAT} = {\left\lbrack {{{{SQRT}\left( a_{1} \right)}*T_{A}} + {\left( {N_{A} + a_{2}} \right)/\left( {2*{{SQRT}\left( a_{1} \right)}} \right)}} \right\rbrack^{2} + {\left. \quad{\left\lbrack {{{{SQRT}\left( b_{1} \right)}*T_{B}} + N_{B} + b_{2}} \right)/\left( {2*{{SQRT}\left( b_{1} \right)}} \right)} \right\rbrack 2} + a_{3} + b_{3} + P_{DC\_ LOAD} - {\left( {N_{A} + a_{2}} \right)^{2}/\left( {4*a_{1}} \right)} - {\left( {N_{B} + b_{2}} \right)^{2}/\left( {4*b_{1}} \right)}}} & \lbrack 10\rbrack \end{matrix}$

This reduces to Eq. 11 as follows.

P _(BAT)=(A ₁ *T _(A) +A ₂)²+(B ₁ *T _(B) +B ₂)² +C   [11]

wherein A₁=SQRT(a₁),

B₁=SQRT(b₁),

A₂=(N_(A)+a₂)/(2*SQRT(a₁)),

B₂=(N_(B)+b₂)/(2*SQRT(b₁)), and

C=a₃+b₃+P_(DC) _(—) _(LOAD)−(N_(A)+a₂)²/(4*a₁)−(N_(B)+b₂)²/(4*b₁)

The motor torques T_(A) and T_(B) can be transformed to T_(X) and T_(Y) as follows:

$\begin{matrix} {\begin{bmatrix} T_{X} \\ T_{Y} \end{bmatrix} = {{\begin{bmatrix} A_{1} & 0 \\ 0 & B_{1} \end{bmatrix}*\begin{bmatrix} T_{A} \\ T_{B} \end{bmatrix}} + \begin{bmatrix} A_{2} \\ B_{2} \end{bmatrix}}} & \lbrack 12\rbrack \end{matrix}$

wherein T_(X) is the transformation of T_(A),

T_(Y) is the transformation of T_(B), and

A₁, A₂, B₁, B₂ comprise application-specific scalar values.

Eq. 11 can thus be further reduced as follows.

P _(BAT)=(T _(X) ² +T _(Y) ²)+C   [13]

P _(BAT) =R ² +C   [14]

Eq. 12 specifies the transformation of motor torque T_(A) to T_(X) and the transformation of motor torque T_(B) to T_(Y). Thus, a new coordinate system referred to as T_(X)/T_(Y) space is defined, and Eq. 13 comprises battery power, P_(BAT), transformed into T_(X)/T_(Y) space. Therefore, the battery power range between maximum and minimum battery power P_(BAT) _(—) _(MAX) and P_(BAT) _(—) _(MIN) can be calculated and graphed as radii R_(Max) and R_(Min) with a center at locus (0, 0) in the transformed space T_(X)/T_(Y), designated by the letter K as shown with reference to FIG. 3, wherein:

R _(Min) =SQRT(P _(BAT) _(—) _(MIN) −C), and

R _(Max) =SQRT(P _(BAT) _(—) _(MAX) −C).

The minimum and maximum battery powers, P_(BAT) _(—) _(MIN) and P_(BAT) _(—) _(MAX), are preferably correlated to battery physics, e.g. state of charge, temperature, voltage and usage (amp-hour/hour). The parameter C, above, is defined as the absolute minimum possible battery power at given motor speeds, N_(A) and N_(B), within the motor torque limits. Physically, when T_(A)=0 and T_(B)=0 the output power from the first and second electric machines 56 and 72 is zero. Physically T_(X)=0 and T_(Y)=0 corresponds to a maximum charging power for the ESD 74. The positive sign (‘+’) is defined as discharging power from the ESD 74, and the negative sign (‘−’) is defined as charging power into the ESD 74. R_(Max) defines a maximum battery power, typically a discharging power, and R_(Min) defines a maximum battery charging power.

The forgoing transformations to the T_(X)/T_(Y) space are shown in FIG. 3, with representations of the battery power constraints as concentric circles having radii of R_(Min) and R_(Max) (‘Battery Power Constraints’) and linear representations of the motor torque constraints (‘Motor Torque Constraints’) circumscribing an allowable operating region. Analytically, the transformed vector [T_(X) T_(Y)] determined in Eq. 12 is solved simultaneously with the vector defined in Eq. 13 comprising the minimum and maximum battery powers identified by R_(Min) and R_(Max) to identify a range of allowable torques in the T_(X)/T_(Y) space which are made up of motor torques T_(A) and T_(B) constrained by the minimum and maximum battery powers P_(BAT) _(—) _(MIN) to P_(BAT) _(—) _(MAX). The range of allowable torques in the T_(X)/T_(Y) space is shown with reference to FIG. 3, wherein points A, B, C, D, and E represent the bounds, and lines and radii are defined.

A constant torque line can be defined in the T_(X)/T_(Y) space, and depicted in FIG. 3 (‘T_(M1)=C1’), comprising the limit torque T_(M1), described in Eq. 1, above. The limit torque T_(M1) comprises the output torque T_(O) in this embodiment, Eqs. 1, 2, and 3 restated in the T_(X)/T_(Y) space are as follows.

T _(M1) =T _(A) to T _(M1)*(T _(X) −A ₂)/A ₁ +T _(B) to T _(M1)*(T _(Y) −B ₂)/B ₁+Misc_(—) T _(M1)   [15]

T _(M2) =T _(A) to T _(M2)*(T _(X) −A ₂)/A ₁ +T _(B) to T _(M2)*(T _(Y) −B ₂)/B ₁+Misc_(—) T _(M2)   [16]

T _(M3) =T _(A) to T _(M3)*(T _(X) −A ₂)/A ₁ +T _(B) to T _(M3)*(T _(Y) −B ₂)/B ₁+Misc_(—) T _(M3)   [17]

Defining T_(M1) _(—) _(XY), T_(M2) _(—) _(XY), and T_(M3) _(—) _(XY) as parts of T_(M1,) T_(M2,) and T_(M3,) contributed by T_(A) and T_(B) only, then:

T _(M1) _(—) _(XY) =T _(A) to T _(M1)*(T _(X) −A ₂)/A ₁ +T _(B) to T _(M1)*(T _(Y) −B ₂)/B ₁   [18]

T _(M2) _(—) _(XY) =T _(A) to T _(M2)*(T _(X) −A ₂)/A ₁ +T _(B) to T _(M2)*(T _(Y) −B ₂)/B ₁   [19]

T _(M3) _(—) _(XY) =T _(A) to T _(M3)*(T _(X) −A ₂)/A ₁ +T _(B) to T _(M3)*(T _(Y) −B ₂)/B ₁   [20]

The following coefficients can be defined:

-   T_(X) to T_(M1)=T_(A) to T_(M1)/A₁, -   T_(Y) to T_(M1)=T_(B) to T_(M1)/B₁, -   T_(M1) _(—) Intercept=T_(A) to T_(M1)*A₂/A₁+T_(B) to T_(M1)*B₂/B₁, -   T_(X) to T_(M2)=T_(A) to T_(M2)/A₁, -   T_(Y) to T_(M2)=T_(B) to T_(M2)/B₁, -   T_(M2) _(—) Intercept=T_(A) to T_(M2)*A₂/A₁+T_(B) to T_(M2)*B₂/B₁, -   T_(X) to T_(M3)=T_(A) to T_(M3)/A₁, -   T_(Y) to T_(M3)=T_(B) to T_(M3)/B₁, and -   T_(M3) _(—) Intercept=T_(A) to T_(M3)*A₂/A₁+T_(B) to T_(M3)*B₂/B₁.

Thus, Eqs. 1, 2, and 3 are transformed to T_(X)/T_(Y) space as follows.

T _(M1) _(—) _(XY) =T _(X) to T _(M1) *T _(X) +T _(Y) to T _(M1) *T _(Y) +T _(M1) Intercept   [21]

T _(M2) _(—) _(XY) =T _(X) to T _(M2) *T _(X) +T _(Y) to T _(M2) *T _(Y) +T _(M2) _(—) Intercept   [22]

T _(M3) _(—) _(XY) =T _(X) to T _(M3) *T _(X) +T _(Y) to T _(M3) *T _(Y) +T _(M3) _(—) Intercept   [23]

The speed constraints, motor torque constraints, and battery power constraints can be determined during ongoing operation and expressed in linear equations which are transformed to T_(X)/T_(Y) space. Eq. 21 comprises a limit torque function describing the output torque constraint T_(M1), e.g., T_(O).

The torque limit of the transmission 10, in this embodiment the output torque T_(O), can be determined by using Eq. 21 subject to the T_(M2) and T_(M3) constraints defined by Eqs. 22 and 23 to determine a transformed maximum or minimum limit torque in the T_(X)/T_(Y) space, comprising one of T_(M1) _(—) _(XY)Max and T_(M1) _(—) _(XY)Min, e.g., maximum and minimum output torques T_(O) _(—) _(Max) and T_(O) _(—) _(Min) that have been transformed. Subsequently the transformed maximum or minimum limit torque in the T_(X)/T_(Y) space can be retransformed out of the T_(X)/T_(Y) space to determine maximum or minimum limit torques T_(M1) _(—) Max and T _(M1) _(—) Min for managing control and operation of the transmission 14 and the first and second electric machines 56 and 72.

FIG. 4 shows motor torque constraints comprising the minimum and maximum motor torques for T_(A) and T_(B) transformed to T_(X)/T_(Y) space (‘Tx_Min’, ‘Tx_Max’, ‘Ty_Min’, ‘Ty_Max’). Battery power constraints are transformed to the T_(X)/T_(Y) space (‘R_Min’, ‘R_Max’) and have a center locus point K comprising (Kx, Ky)=(0,0). The output torque constraint (Tm1=−Tx+Ty) is shown, having a tangent point with the battery power constraint.

Constraints comprising maximum and minimum limits for a first torque input to the transmission 10 are depicted (‘Tm2=Tm2_High_Lmt’ and ‘Tm2=Tm2_Low_Lmt’), and preferably comprise the range of input torques T_(I) at input shaft 12 transformed to T_(X)/T_(Y) space in this embodiment and can be mathematically represented by the line T_(M2) _(—) _(XY) described with reference to Eq. 22, above. The lines T_(M2) _(—) _(XY) described in Eq. 22 include the T_(M2) _(—) Intercept having two different values corresponding to the maximum limit and the minimum limit for the engine input torque T_(I). Alternatively, the second input torque T_(M2) _(—) _(XY) can comprise a range of clutch torques or another torque input.

Constraints comprising maximum and minimum limits for a second torque input to the transmission 10 are depicted (‘Tm3_High_Lmt’) and a low limit (‘Tm3_Low_Lmt’), and preferably comprise the range of applied clutch torques transformed to T_(X)/T_(Y) space in this embodiment and can be mathematically represented by the line T_(M3) _(—) _(XY) described with reference to Eq. 23, above. The lines T_(M3) _(—) _(XY) described in Eq. 23 include the T_(M3) _(—) Intercept having two different values corresponding to the maximum limit and the minimum limit for the applied one of the torque-transfer clutches C1 70, C2 62, C3 73, C4 75 of the transmission 10. Alternatively, the second input torque T_(M2) _(—) _(XY) can comprise a range of engine input torque or another torque input.

The output torque line (‘Tm1=−Tx+Ty’) representing line T_(M1) _(—) _(XY) has a positive slope of a/b of the general form in Eq. 24:

Tm1=a*Tx+b*Ty+C   [24]

wherein a<0 and b>0 and C is a constant term. In the ensuing descriptions, the line T_(M1) _(—) _(XY) has a positive slope of 1:1 for purposes of illustration. The x-intercept C of Eq. 24 can change. The output torque line comprises the limit torque function describing the output torque.

FIG. 5 depicts a process for determining one of the maximum and minimum output torques T_(O) _(—) _(Max) and T_(O) _(—) _(Min) based upon the speed constraints, motor torque constraints, and battery power constraints, and the first torque input range and the second torque input range, with reference back to FIG. 4. Equations for the maximum and minimum output torques, the speed constraints, the motor torque constraints, the battery power constraints, and the first torque input range and the second torque input range are transformed into T_(X)/T_(Y) space. The maximum and minimum output torques T_(O) _(—) _(MAX) and T_(O) _(—) _(MIN) comprise one of T_(M1) _(—) _(XY)Max and T_(M1) _(—) _(XY)Min. It is determined whether the preferred solution is a maximum value for the output torque, i.e., T_(M1) _(—) _(XY)Max as indicated by setting a flag (‘Tm1_Max_Flag’), or alternatively whether the preferred solution is a minimum value for the output torque, i.e., T_(M1) _(—) _(XY)Min as indicated by not setting the Tm1_Max_Flag flag. A maximum (or minimum) value for the output torque Tm1 is calculated based upon the motor torque constraints and battery power constraints in T_(X)/T_(Y) space, comprising one of T_(M1) _(—) _(XY)Max and T_(M1) _(—) _(XY)Min, and having coordinates of (Tx, Ty) (505).

A value for the second input torque T_(M2) _(—) _(XY) (‘Tm2_Value’) is calculated at the maximum (or minimum) value for the output torque Tm1 and having coordinates of (Tx, Ty), using Eq. 22 (530). It is determined whether the calculated value for the second input torque T_(M2) _(—) _(XY) (‘Tm2_Value’) is within the operating range of the second input torque, shown in FIG. 4 as the lines representing the maximum and minimum limits for the first torque input to the transmission 10 (‘Tm2=Tm2_High_Lmt’ and ‘Tm2=Tm2_Low_Lmt’) (532).

When it is determined that the calculated value for the second input torque T_(M2) _(—) _(XY) (‘Tm2_Value’) is within the operating range of the second input torque at the maximum (or minimum) value for the first torque Tm1, an initially achievable first torque point having coordinates of (Tx1, Ty1) is set equal to (Tx, Ty) (536, 540). When it is determined that the calculated value for the second input torque T_(M2) _(—) _(XY) (‘Tm2_Value’) is outside the operating range of the second input torque at the maximum (or minimum) value for the output torque Tm1, the (Tx, Ty) point is modified to an initially achievable first torque point having coordinates of (Tx1, Ty1) (534, 538). The initially achievable first torque point lies on one of the lines representing the maximum and minimum limits for the first torque input to the transmission 10, depending on the calculated value for the second input torque T_(M2) _(—) _(XY) at (Tx, Ty). In either instance, a maximum (or minimum) value for the output torque Tm1 is calculated at the initially achievable first torque point having the coordinates of (Tx1, Ty1) using Eq. 21 (542).

A value for the third input torque T_(M3) _(—) _(XY) (‘Tm3_Value’) is calculated at the maximum (or minimum) value for the first torque Tm1 and having coordinates of (Tx, Ty), using Eq. 23 (510). It is determined whether the calculated value for the third input torque T_(M3) _(—) _(XY) (‘Tm3_Value’) is within the operating range of the third input torque, shown in FIG. 4 as the lines representing the high limit (‘Tm3_High_Lmt’) and the low limit (‘Tm3_Low_Lmt’) (512). When it is determined that the calculated value for the third input torque T_(M3) _(—) _(XY) (‘Tm3_Value’) is within the operating range of the third input torque at the maximum (or minimum) value for the output torque Tm1, an initially achievable second torque point having coordinates of (Tx2, Ty2) is set equal to (Tx, Ty) (516, 520). When it is determined that the calculated value for the third input torque T_(M3) _(—) _(XY) (‘Tm3_Value’) is outside the operating range of the third input torque at the maximum (or minimum) value for the output torque Tm1, the (Tx, Ty) point is modified to an initially achievable second torque point having coordinates of (Tx2, Ty2) (514, 518). The initially achievable first torque point lies on one of the lines representing the maximum and minimum limits for the second torque input to the transmission 10, depending on the calculated value for the third input torque T_(M3) _(—) _(XY) at (Tx, Ty). In either instance, a maximum (or minimum) value for the output torque Tm1 is calculated at the initially achievable second torque point having the coordinates of (Tx2, Ty2) using Eq. 21 (522).

The initially achievable first torque point having the coordinates of (Tx1, Ty1) and the initially achievable second torque point having the coordinates of (Tx2, Ty2) are compared (550). When the preferred solution is a maximum value for the output torque, i.e., T_(M1) _(—) _(XY)Max as indicated by setting the flag (‘Tm1_Max_Flag’) and the initially achievable first torque point having the coordinates of (Tx1, Ty1) is greater than or equal to the initially achievable second torque point having the coordinates of (Tx2, Ty2), then the final solution (Tx_Final, Ty_Final) is the output torque at the achievable first torque point having the coordinates of (Tx1, Ty1) (550, 570).

When the preferred solution is a maximum value for the output torque, i.e., T_(M1) _(—) _(XY)Max as indicated by setting the flag (‘Tm1_Max_Flag’) and the initially achievable first torque point having the coordinates of (Tx1, Ty1) is less than the initially achievable second torque point having the coordinates of (Tx2, Ty2), then the final solution (Tx_Final, Ty_Final) is the output torque at the achievable second torque point having the coordinates of (Tx2, Ty2) (550, 560).

When the preferred solution is a minimum value for the output torque, i.e., T_(M1) _(—) _(XY)Min as indicated by not setting the flag (‘Tm1_Max_Flag’) and the initially achievable first torque point having the coordinates of (Tx1, Ty1) is less than or equal to the initially achievable second torque point having the coordinates of (Tx2, Ty2), then the final solution (Tx_Final, Ty_Final) is the achievable first torque point having the coordinates of (Tx1, Ty1) (550, 570). When the preferred solution is a minimum value for the output torque, i.e., T_(M1) _(—) _(XY)Min as indicated by not setting the flag (‘Tm1_Max_Flag’) and the initially achievable first torque point having the coordinates of (Tx1, Ty1) is greater than the initially achievable second torque point having the coordinates of (Tx2, Ty2), then the final solution (Tx_Final, Ty_Final) is the output torque at the achievable second torque point having the coordinates of (Tx2, Ty2) (550, 560).

The (Tx_Final, Ty_Final) point represents the preferred solution for controlling operation that can be retransformed to motor torques (T_(A), T_(B)) to control operation of the first and second electric machines 56 and 72 to achieve the output torque (580). Thus, the preferred minimum or maximum output torque is constrained based upon the speed constraints, the motor torque constraints, the battery power constraints, and the first torque input range, e.g., the input torque T_(I), and the second torque input range, e.g., the clutch torque.

The embodiment described hereinabove is based upon the output torque line T_(M1) _(—) _(XY) having a positive slope of a/b of the general form in Eq. 24 (as above):

Tm1=a*Tx+b*Ty+C   [24]

wherein a<0 and b>0 and C is a constant term, with a slope of a/b=1:1 for purposes of illustration with the x-intercept C being changeable, and is indicative of a maximum battery power discharge. The description is applicable to combinations of a>0, b<0, and the slope of a/b being less than 1:1 and being greater than 1:1.

The disclosure has described certain preferred embodiments and modifications thereto. Further modifications and alterations may occur to others upon reading and understanding the specification. Therefore, it is intended that the disclosure not be limited to the particular embodiment(s) disclosed as the best mode contemplated for carrying out this disclosure, but that the disclosure will include all embodiments falling within the scope of the appended claims. 

1. A method for controlling an electro-mechanical transmission operatively coupled to first and second electric machines to transmit power to an output member, the method comprising: determining motor torque constraints for the first and second electric machines; determining battery power constraints for an electrical energy storage device electrically connected to the first and second electric machines; determining a range for a first torque input to the electro-mechanical transmission; determining a range for a second torque input to the electro-mechanical transmission; and determining a preferred output torque to the output member of the electro-mechanical transmission that is achievable within the motor torque constraints, is achievable within the range for the first torque input, is achievable within the range for the second torque input, and is based upon the battery power constraints.
 2. The method of claim 1, wherein the preferred output torque comprises a maximum tractive torque to the output member.
 3. The method of claim 1, wherein the preferred output torque comprises a maximum braking torque to the output member.
 4. The method of claim 1, wherein the first torque input comprises an engine input torque.
 5. The method of claim 4, wherein the second torque input comprises a torque for an applied clutch.
 6. The method of claim 1, further comprising controlling operation of the electro-mechanical transmission and the first and second electric machines to achieve the preferred output torque at the output member that is achievable within the motor torque constraints, is achievable within the range for the first torque input, is achievable within the range for the second torque input, and is based upon the battery power constraints.
 7. The method of claim 1, wherein the preferred output torque comprises a commanded output torque to the output member to achieve an operator torque request.
 8. The method of claim 1, wherein the battery power constraint comprises a maximum battery discharging power and a maximum battery charging power.
 9. The method of claim 1, further comprising: determining an initially achievable first torque point achievable within the motor torque constraints, achievable within the range for the first torque input, and based upon the battery power constraints; determining an initially achievable second torque point achievable within the motor torque constraints, achievable within the range for the second torque input, and based upon the battery power constraints; and selecting the preferred output torque to the output member based upon the initially achievable first torque point and the initially achievable second torque point.
 10. The method of claim 1, further comprising: formulating mathematical equations representing maximum and minimum motor torque constraints for the first and second electric machines and representing maximum and minimum battery power constraints; formulating a mathematical equation representing the output torque; formulating mathematical equations representing the range for the first torque input; formulating mathematical equations representing the range for the second torque input; transforming the mathematical equations representing the maximum and minimum battery power constraints to equations of concentric circles having respective radii; transforming the mathematical equations representing the maximum and minimum motor torque constraints for the first and second electric machines to equations consisting of lines; transforming the mathematical equations representing the range for the first torque input to equations consisting of lines; transforming the mathematical equations representing the range for the second torque input to equations consisting of lines; and transforming the mathematical equation representing the output torque to an equation consisting of a line.
 11. The method of claim 10, further comprising: determining at least one transformed achievable output torque based upon the transformed motor torque constraints for the first and second electric machines, the transformed battery power constraints, the transformed range for the first torque input, and the transformed range for the second torque input; determining a transformed achievable maximum output torque from the electro-mechanical transmission; and retransforming the transformed achievable maximum output torque to determine preferred motor torques for the first and second electric machines.
 12. The method of claim 10, comprising calculating at least one intersection of the transformed mathematical equations representing the maximum and minimum battery power constraints, the transformed mathematical equations representing the maximum and minimum motor torque constraints for the first and second electric machines, the transformed mathematical equation representing the first and second torque inputs, and the transformed mathematical equation representing the output torque.
 13. A method for controlling an electro-mechanical transmission operatively coupled to first and second electric machines to transmit mechanical power to an output member, the method comprising: determining motor torque constraints for the first and second electric machines; determining battery power constraints for an electrical energy storage device electrically connected to the first and second electric machines; determining a range for a first torque input to the electro-mechanical transmission; determining a range for a second torque input to the electro-mechanical transmission; determining a preferred output torque to the output member of the electro-mechanical transmission that is achievable within the motor torque constraints, is achievable within the range for the first torque input, is achievable within the range for the second torque input, and is based upon the battery power constraints. 